1. Field of the Invention
The present invention relates to an ink jet recording apparatus provided with a discharge recovery systems to eliminate from the discharging member the defective discharges brought about by the clogging due to dust particles and the increase of viscosity of ink, or the mixture of air bubbles or the like.
2. Related Background Art
FIG. 1 is a perspective view showing an example of the recording head used for an ink jet recording apparatus according to the prior art. Here, a reference numeral 1 designates a discharging element comprising the ink passages in which the heat generating elements composed of HfB.sub.2 and others are integrally arranged in parallel for enabling discharging energy to act on the recording liquid (hereinafter referred to as ink); the open orifices 10 which are provided in the front part of each of the ink passages; and a common liquid chamber where the ink is retained for supply to each of the ink passages, among some others. Thus, the ink droplets are discharged from the orifices to execute an image formation. A reference numeral 3 designates a base plate on which the element 1 is adhesively bonded, and 2, a front plate fixed by bolts or other tightening members to the end faces of the discharging element 1 and base plate 3, having the apertures which enable the orifices 10 to face a recording medium such as a recording sheet directly. Each of the members 15, 16, and 17 is the one constituting a part of the ink supply systems; 15 is an elbow type connecting member which leads the ink into the common liquid chamber in the discharging element; 17 is a filter unit arranged on the way of the ink supply passages from an ink tank or the like which serves as an ink supply source; and 16 is a supply tube connecting between the connecting member 15 and the filter unit 17.
FIG. 2 and FIG. 3 are cross-sectional views taken along the vertical and horizontal planes of the recording head shown in FIG. 1, respectively, which represent a state where a cap 4 is coupled to cover the entire surface of the aperture plane of the orifices 10 of the discharging element through the front plate 2 when a discharge recovery processing is executed.
The ink passages 12 corresponding to a plurality of orifices 10 are connected to the so-called eave portion or appentice cave portions 13. The appentice cave portions 13 are connected to the common liquid chamber 14. A reference numeral 11 designates an energy generating element provided in the ink passage 12 to cause the discharging energy to act on the ink, which is formed by a heat generating element, for example. In the filter unit 17, a filter 100 composed of ridge meshes, for example, is provided in order to remove fine dust particles and air bubbles.
FIG. 4 is a block diagram showing the discharge recovery systems in an ink jet recording apparatus according to the prior art. In a usual recording state, the cap 4 is set in an appropriate position where it does not hinder any recording operation. At the same time, a valve B2 is opened, while valves B1 and B3 are kept in the closed condition. Ink is supplied from the ink tank 6 to the discharging element 1 by the application of the capillary phenomenon through the valve B1. When a discharge recovery operation is executed, the cap 4 is installed on the discharging element 1. Then, at first, the valves B1, B2, and B3 are kept in the open state. In this state, a pump 7 is driven to send the ink from the ink tank 6 into the ink supply passages under pressure, thus circulating it through the ink tank 6.fwdarw.ink supply passage L.sub.1 .fwdarw.discharging element 1.fwdarw.ink supply passage L.sub.2 .fwdarw.ink tank in that order to remove air bubbles in the ink supply passages L.sub.1 and L.sub.2 as well as in the discharging element 1. Then, by closing the valve B1, the pressurized ink is supplied to the discharging element 1 so that the ink is forcibly discharged from the orifices 10.
At this juncture, the fine dust particles, the over viscous ink resulting from the evaporation of the ink solvent, and the air bubbles contained in the ink, which are among those causes of the defective discharge, are all exhausted from the discharging element 1 together with the ink discharged from the orifices. For example, as shown in FIG. 3, the ingressive fine air bubbles a in the ink passages 12 are removed from the orifices 10 together with the ink when the pump 7 is driven. The ink thus discharged from the orifices 10 are received by the cap 4 and led out to a waste ink tank 5.
FIG. 5 is a diagram showing the hydrodynamic equivalent circuit for ink in the conventional. apparatus at the time of discharge recovery operation. Here, given the pressure exerted by the pump 7 as .DELTA.P; the number of nozzles, as n; the fluid resistance per nozzle of the nozzles 12, as R1; fluid resistance in the appentice caves 13, as RH; fluid resistance in the common liquid chamber 14, as RC; fluid resistance in the filter unit 17, as RF; fluid resistance in any portions between the ink tank 6 to the common liquid chamber 14 other than the filter unit 17, as RS; and flow rate per one nozzle of the nozzles 12 when pressurized by the pressure .DELTA.P, as q, the following relationship will be satisfied at the time of discharge recovery operation: EQU .DELTA.P=qR1+nq (RH+RC+RF+RS) (1) EQU .thrfore.q=.DELTA.P/{R1+n (RH+RC+RF+RS)} (2)
Usually, the supply systems are designed to obtain the relationship of R1&gt;&gt;RH+RC+RF+RS. However, in a recording head having a mode that the nozzles 12 are arranged in a number corresponding to the recording width, that is, the so-called full-multi type recording head, the number of the nozzles n becomes great, and the flow rate per nozzle q becomes extremely small. Also, as shown in FIG. 8, if there exist ingressive air bubbles a, dust particles, and the like in the nozzles 12, the fluid resistance R1 in the nozzles in which the air bubbles or others exist becomes high. Therefore, the flow rate to the nozzle where the defective discharge takes place becomes still smaller than to the nozzle in the normal state.
As a result, when the conventional recovery systems are employed, the nozzle which has brought about the defective discharge may not be restored to the normal condition, or the discharge recovery processing should be repeated before it is restored.
Also, it is necessary to make the pressure greater in order to overcome such a fluid resistance as this. Hence, there may be a need for the provision of a greater pump 7, leading to the increased consumption of ink, and still more, to the enhancement of the strength of each connecting part between the members to enable them to withstand the increased pressure.
Also, in the conventional discharge recovery systems, although the air bubbles, dust particles, and others in the nozzles can be removed, it is still difficult to completely remove the dust particles and ink droplets adhering to the circumference of the apertures for discharging.
Meanwhile, the filter 100 provided for the ink supply passage is arranged to prevent the dust particles from being ingressive into the recording head 1 (the common liquid chamber 14 and ink passages 12) from the ink supply systems. The smaller the meshes of a filter of the kind, its effectiveness is greater, but the smaller the meshes of the filter, the greater becomes the fluid resistance RF in the filter portion. Thus, the pressure loss will become greater at the time of discharging the ink, leading to the increased frequency of the defective discharge.
To counteract this, it may be attempted to select the filter with a view to avoiding the defective discharges in the entire discharging ports while reducing the pressure loss. In this case, however, the required area of the filter becomes great particularly when the full-multi head having many numbers of discharging ports should be arranged. Therefore, the recording head 1 must inevitably be large in its size as the structure requires the filter to be arranged for the recording head 1 (in the common liquid chamber 14). This is a disadvantage encountered in this respect. Also, in a structure that the filter is installed on the way of the ink passage, it should be possible to adopt a filter having a large area comparatively easily, but there is still a disadvantage that the handling of the recording head 1 becomes inconvenient because it is necessary to keep the ink supply tube connected to the recording head 1 at all times. Moreover, the problems are encountered in that the assembling operation becomes difficult in the process of fabricating the recording head, and that there is a possibility that the dust particles can enter after the replacement of the heads if such a type is adopted as to make the detachment and replacement possible individually on the ink supply side and the recording head side.